Revisiting hotspots and continental breakup—Updating the classical three-arm model

ABSTRACT Classic models proposed that continental rifting begins at hotspots—domal uplifts with associated magmatism—from which three rift arms extend. Rift arms from different hotspots link up to form new plate boundaries, along which the continent breaks up, generating a new ocean basin and leaving failed arms, termed aulacogens, within the continent. In subsequent studies, hotspots became increasingly viewed as manifestations of deeper upwellings or plumes, which were the primary cause of continental rifting. We revisited this conceptual model and found that it remains useful, though some aspects require updates based on subsequent results. First, the rift arms are often parts of boundaries of transient microplates accommodating motion between the major plates. The microplates form as continents break up, and they are ultimately incorporated into one of the major plates, leaving identifiable fossil features on land and/or offshore. Second, much of the magmatism associated with rifting is preserved either at depth, in underplated layers, or offshore. Third, many structures formed during rifting survive at the resulting passive continental margins, so study of one can yield insight into the other. Fourth, hotspots play at most a secondary role in continental breakup, because most of the associated volcanism reflects plate divergence, so three-arm junction points may not reflect localized upwelling of a deep mantle plume.

Contrasting transform and passive margin subsidence history and heat flow evolution: insights from 3D thermo-mechanical modelling

Transform and passive margins developed during the continental rifting and opening of oceanic basins are fundamental elements of plate tectonics. It has been suggested that inherited structures, plate divergence velocities and surface processes exert a first order control on the topographic and bathymetric evolution and thermal history of these margins and related sedimentary basins. Their complex spatial-temporal dynamics have remained controversial. Here, we conducted 3D magmatic-thermo-mechanical numerical experiments coupled with surface processes modelling to simulate the dynamics of continental rifting, continental transform fault zone formation and persistent oceanic transform faulting and zero-offset oceanic fracture zones development. Numerical modelling results allow to explain the first order observations from passive and transform margins, such as diachronous rifting, heat flow rise and cooling in individual depocenters and contrasting basin tectonics of extensional and transtensional origin. In addition, the models reproduce the rise of both marginal ridges and transform marginal plateaus and their interaction with erosion and sedimentation. Comparison of model results with observations from natural examples yield new insights into the tectono-sedimentary and thermal evolution of several key passive and transform continental margins worldwide.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5756555

The evolution of triple junctions: from failure to success

Divergent triple junctions are stable plate margins where three spreading ridges meet. Although it is accepted that this configuration is inherited from an earlier phase of continental rifting, how post-breakup triple junctions emerge from the separation of two plates remains unclear. By documenting the strain rate history recorded in the three rift-arms of several modern and ancient triple junctions, we show that deformation is episodic and localized in only one or two rifts at any given time. We further investigate this behavior in three-dimensional (3D) analog experiments of rifting, under a range of kinematic boundary conditions and containing a variety of pre-existing lithospheric heterogeneities. Deformation in the experiments is characterized by strain jumps and rift abandonment, comparable to natural observations. Boundary rotation during extension induces oblique stretching directions, along-strike strain gradients and forces significant strain jump to reduce the number of rifts segments active. Models that comprise lithospheres ranging from homogenous to containing a triple junction-like pre-existing heterogeneities, never developed a three-armed rift, where all rift segments are active at same time, at any stage. Our experimental results indicate that, unlike mature, successful, and stable oceanic triple junctions, early-stage continental rifting progresses through unstable “double-junctions” characterized by repeated strain jumps and rift failures and reactivations.